1. Technical Field
This invention relates to a receiver including two tuners, which decodes digital data contained in electromagnetic signals, such as radio broadcast signals. Further, this invention relates to a method for decoding digital data in such signals.
2. Related Art
Worsening traffic congestion has generated a demand for accurate and timely information on traffic conditions. Broadcasters, such as radio broadcasters, cannot respond simply by increasing the number of on-air announcements that are broadcast using traditional delivery modes, because this would be unacceptable to listeners. Many listeners are concerned that such announcements are untrustworthy, repetitive and irritating unless the information is relevant to them. The transmission of traffic and travel information (“TTI”) using data broadcast methods potentially solves this problem because the TTI data can be transmitted and decoded inaudibly, and filtered by a receiver to help ensure that listeners only obtain data that is relevant to their journey.
Multiple delivery methods of TTI services have emerged. Examples of TTI delivery methods include: digital audio broadcast (“DAB”), digital video broadcasting terrestrial (“DVBT”), the global system for mobile communications (“GSM”), the universal mobile telecommunication system (“UMTS”), and the radio data system (“RDS”). In addition, approaches to deliver this information via the Internet have been proposed. Because RDS has been widely deployed in Europe and elsewhere, car manufactures have integrated radio receivers for vehicles with multi-functional displays in vehicle dashboards. Often, these integrated systems further include modules such as telematic devices or a navigation system.
RDS uses a technique of adding data to a sub-carrier on an existing stereo transmission in such a way that the data may be decoded inaudibly. RDS is designed with a wide range of features that support program-related services, and allow non-program related data services to be added, if the capacity exists. One service supported by the RDS features is the alternative frequency (“AF”) service. The AF service enables receivers to scan the radio frequency band to find the frequency at which a particular program may be received most clearly. In general, the AF service communicates the frequencies at which various transmitters broadcast the same program in the same or adjacent physical areas (the “alternative frequencies”). This enables receivers equipped with a memory to store a list of the alternative frequencies, thereby reducing the time needed to switch frequencies when scanning for a clearer transmission. Another service supported by the RDS features is the program service (“PS”) name feature. The PS name feature enables an RDS receiver to display the name of the program, instead of, for example, the tuning frequency.
Other services that may be supported by the RDS features include the traffic announcement (“TA”) and traffic program (“TP”) services, which inform a receiver as to whether the signal being received carries traffic announcements. Further services supported by the RDS features include program type (“PTY”), radio text (“RT”), clock type and date (“CT”), decoder identification (“DI”), dynamic PTY indicator (“PTYI”), enhanced other networks information (“EON”), music speech switch (“MS”), open data applications (“ODA”) and program item number (“PIN”) services. Another service enabled by RDS is the traffic message channel (“TMC”). The objective of the RDS TMC is to broadcast digital traffic and TTI messages as data in an FM transmission of a program so that these messages may be decoded inaudibly by a receiver. The digital traffic and TTI messages may be communicated to a user via a visual interface or display. This allows delivery of accurate, timely and relevant information without the need to interrupt the program.
A receiver used to decode electromagnetic signals, such as radio broadcast signals that include a program and digital data, may include a main tuner and a background tuner. The main tuner is usually responsible for demodulating the program, such as a broadcasted radio broadcast program, and for providing the demodulated signal to an interface such as a speaker for conversion into sound waves. Further, the main tuner may be used to demodulate and decode digital data relating to the program being demodulated. For example, the data may be RDS data. In contrast, the primary task of the background tuner is to scan the frequency band and decode other electromagnetic signals at other frequencies that include the same program as that being demodulated by the main tuner (“alternative frequencies”). Commonly, the alternative frequencies are identified within the RDS data stream (often using the AF feature) included along with the program in the signal demodulated by the main tuner. Therefore, the RDS data stream may be decoded by the main tuner and communicated to the background tuner. The background tuner may then analyze the RDS data stream to obtain the alternative frequencies. In addition, the main tuner may be used to decode other types of RDS data configured in data blocks, such as RDS TMC data. It is also possible for the background tuner to decode such data blocks. In both solutions, the main tuner and the background tuner may be tuned to a fixed frequency.